Which Bt Corn Works Best?

If you're a farmer debating which Bt corn hybrid to plant in '07, insect protection should be about the same no matter which brand you choose — unless you're trying to control an insect other than corn borer or corn rootworm. Bt technologies that target these two corn pests provide virtually equal control, according to several extension entomologists across the U.S.

“There are essentially three different Bt events aimed at corn borer,” says Phil Sloderbeck, Kansas State University Southwest Area Extension entomologist. “There's Agrisure, YieldGard and Herculex, and they all do a really good job.”

All Bt corn hybrids have been genetically engineered to produce a protein in their plant tissue that is toxic to certain insect pests, but not to livestock or humans. This protein is derived from a common soil microbe, Bacillus thuringiensis (Bt).

For the 2006 growing season, only two Bt technologies had EPA approval for corn rootworm control — Herculex and YieldGard, says Sloderbeck. “For rootworm protection, Monsanto's YieldGard RW or YieldGard Plus is fairly similar in control with Herculex I or Herculex XTRA.” A third Bt corn rootworm choice, Agrisure RW, just received EPA approval on Oct. 4, so it's still too early to comment on how it may compare, he adds.

However, Ken Ostlie, a University of Minnesota extension entomologist, says he expects the Agrisure product to be competitive with the other brands.

Still, more studies need to verify any potential differences that may exist among Bt corn rootworm technologies, says Kevin Steffey, University of Illinois Extension entomologist. Steffey agrees with Ostlie and Sloderbeck that protection from the various Bt corn borer events are virtually the same. He also agrees that corn rootworm protection is quite similar among Bt rootworm brands, although some differences have been detected in limited comparison studies.

“This year is the first year that we've had both Herculex and YieldGard products in standard corn rootworm trials,” says Steffey. “YieldGard RW has been the most consistent product in our trials since it became commercially available. It's been more consistent than soil insecticides. On the other hand, at our Urbana location, root protection was significantly better from Herculex than YieldGard this year. Yet, at Monmouth and DeKalb, there was no difference.”

It's too soon to know for certain why a difference occurred at Urbana and not at DeKalb or Monmouth, says Steffey. Whether a difference will occur again at another location or during a different growing season remains to be seen, he adds.

On the other hand, Herculex Bt corn events do provide protection against more insect pests than either YieldGard or Agrisure Bt events, note all three entomologists. “Herculex does have a little better spectrum on some corn pests, such as black cutworm, western bean cutworm, stalk borer and fall armyworm,” says Sloderbeck, “compared to other Bt corn hybrids.”

In the past, control of insects such as the western bean cutworm was either a non-issue or a minor issue in the Eastern Corn Belt, points out Steffey. “Now, the western bean cutworm has greatly expanded its area and is present in Illinois, Indiana, Michigan, Wisconsin and Ohio,” he says. “So, an insect that used to be a non-issue in the East is now a possible issue for management.”

Bt corn hybrid selection can become confusing very quickly unless you focus on which insect or insects you want to protect against, cautions Sloderbeck.

“The key is making sure you know what you're getting, whether it's the corn borer protection alone, the rootworm protection alone or the two stacked together,” he says. “If corn borer is a problem in your area, but you plant just a Bt rootworm hybrid, then you need to figure out how to manage corn borer.”

Agronomic traits are also vital considerations when selecting Bt hybrids. “The first thing is to select a hybrid that is well adapted to your area,” says Sloderbeck. “You need to have a high-yielding variety first. Then, pick the traits that will protect against the major pests in your area.”

For example, Bt rootworm hybrids can be a big advantage in continuous cornfields where rootworms have been a problem in the past, says Sloderbeck. They can also provide a key advantage during dry weather, points out Ostlie.

“Over the last couple of years, a lot of entomologists have been impressed with the yield response from root protection given by Bt rootworm hybrids, particularly during drought,” says Ostlie. “The transgenics are protecting the entire root system better than the soil insecticides.”

Last year, a Bt corn rootworm hybrid performed well for Matt Harguth, who grows continuous corn near Waseca, MN. “It gave us a higher percentage of protection to the roots than soil insecticides,” he says.

However, seed availability, agronomics and price convinced him to stick with soil insecticide applications at planting during 2006, says Harguth. “There wasn't enough seed available with the stacked traits to plant all of it to Bt rootworm and Bt corn borer, so we decided not to plant any Bt rootworm corn this year,” he explains. “We plant both Pioneer and DeKalb Bt corn hybrids, and we decide mostly by what maturity they are and what yields best for our area.”

Bt hybrids with corn borer protection have consistently provided both better yields and an economic payback for Harguth over the years, compared to similar, non-Bt corn hybrids, he says. “Now, we try to plant as close to 80% of our corn to Bt corn borer hybrids as we can,” he says. “The only acres that we don't plant to Bt corn borer are the 20% that we have in our refuge acres.”

The economic payback from transgenic hybrids is important to pencil out before purchasing, agrees Ostlie. “Most farmers don't have a good handle on what risks they're actually facing,” he says. “The threat might not be as much as they think.”

For example, Ostlie says he suspects that populations of corn rootworm beetles can drop considerably over time after repeatedly planting Bt corn rootworm hybrids. “The transgenics have been much better in killing corn rootworm beetles than the soil insecticides ever were,” he says. “We also need to keep in mind that resistance management will be crucial with all these Bt events and to be responsible with their use.”

Transgenic hybrids that can protect against multiple insects may be better reserved for times when farmers are fairly sure that they'll need the protection from all the traits in the seed, agrees Steffey. “Stacked traits won't do anything better than what the individual components would do by themselves for each target pest,” he explains. “Yet, I think a lot of farmers will still be attracted to the stacked traits whether they really need all the traits or not.”

In the past, farmers have preferred using wide, broad-spectrum soil insecticides, notes Steffey. “However, from an integrated pest management and a resistance management standpoint,” he says, “a narrower spectrum of control is better.”

On the other hand, farmers like Harguth find stacked traits an attractive means to avoid insecticide use. “Eventually, we'd like to get away from using any insecticides,” he says. “[Insecticide application] is dusty, dirty and inaccurate a lot of times, because of applicator error, and it's a safety concern that we'd like to avoid.”